Fine by me with BU likely being a fish and all. He could very well have stuff like 88-JJ, he'll show up sometime with KK+ as well not everybody 4-bets premiums for a variety of reasons, mostly bad ones like trapping or just not having a raise button. He could have all sorts of draws if he's capable of playing them aggressively, and if he's awful he can do that with 7x as well.

I might just raise UTG2's donkbet anyway because the board has the potential to get ugly really fast with stuff like 4/8/9/spades on the turn but especially if you know he and/or BU is aggressive it's fine to let them do their thing.